The present invention relates to a power transmission apparatus used for a clutch or a brake, and particularly to a cooling structure thereof.
In an magnetic particle type electromagnetic clutch having an outer member forming an annular space, an inner member arranged inside of the outer member and electromagnetic powder of magnetic particles interposed between the outer and inner members, when an exciting coil supported by the outer member is charged with electricity to produce a magnetic field, magnetic particles in the electromagnetic powder are magnetized and a linkage of the magnetic particles is formed for connecting the outer and inner members. When the exciting coil is not charged with electricity, the outer and inner members are disconnected so as to freely rotate mutually.
In such a magnetic particle type electromagnetic clutch, a large amount of heat is generated by the exciting coil and slipping of the clutch on connecting and disconnecting to deteriorate stable connection-disconnection function and durability, therefore a forced cooling is necessary in general.
Accordingly, in a magnetic particle type electromagnetic clutch disclosed in Japanese Laid-Open Utility Model Publication No. Sho 63-64938 (1988), within an inner member is formed a cooling water passage from a core portion to an outer peripheral portion opposed to an outer member and at the outer peripheral portion is provided a special annular chamber to improve cooling effect.
There is also another example wherein a cooling water passage is formed in an outer member to cool mainly an exciting coil side.
In these examples, the cooling water passage is formed within an inner member or an outer member, so that it is advantageous spatially and the electromagnetic clutch itself does not become large.
However, since the cooling water passage is formed in the inner or outer member, construction of the member becomes complicated and the working is very troublesome. In case of a clutch which is connected and disconnected while both inner and outer members rotate, formation of the cooling water passage at a boundary between a fixed part and a rotating part is difficult. When the cooling water passage is formed in the inner member, cooling effect against heat generation of the exciting coil is small.
Further, the inner and outer members must be worked with high accuracy or sealed surely so that the cooling water passage is blocked surely from the electromagnetic powder and the exciting coil, which are apt to be influenced by liquid, and no leak of the cooling water occurs. This is difficult technically and results in a high cost, sometimes.
When the cooling water passage is formed within the inner or outer member, it is difficult to form fins within a limited interior space resulting in a small heat transmitting area, therefore a large quantity of water must be flowed in order to obtain a high cooling effect. If the cooling water passage is made of iron or steel material apt to rust, there is the possibility that the passage is stopped up by the rust, therefore complications on use such as rust prevention, administration of the cooling liquid, periodical maintenance are accompanied.
Further, when the magnetic particle type electromagnetic clutch is combined with a gear device to constitute a power transmission apparatus and the gear device is also necessitated to be cooled, another cooling apparatus for the gear device has to be provided, that is uneconomical in view of space and cost.